[clang] [CIR][AMDGPU] Add support for AMDGCN rsq builtins (PR #197349)

[Rana Pratap Reddy via cfe-commits]

https://github.com/ranapratap55 approved this pull request.

LGTM with nit. 

Title says only `rsq`. But I see changes for `_clamp` also. Please retitle to add `_clamp`.

https://github.com/llvm/llvm-project/pull/197349